Fusion proteins are polypeptide chains consisting of two or more proteins fused together into a single polypeptide chain. If one of the proteins is a ligand, then the resulting ligand fusion proteins bind to cells bearing receptors specific for the particular ligand.
Where the first protein is a ligand and the second protein is a cytotoxin, the ligand fusion protein may act as a potent cell-killing agent specifically targeting the cytotoxin to cells bearing a particular receptor type. For example, chimeric fusion proteins which include interleukin 4 (IL4) or transforming growth factor (TGF.alpha.) fused to Pseudomonas exotoxin (PE) or interleukin 2 (IL2) fused to Diphtheria toxin (DT) have been tested for their ability to specifically target and kill cancer cells (Pastan et al., Ann. Rev. Biochem., 61: 331-354 (1992)).
Alternatively, where the ligand is fused to another specific binding moiety such as an antibody, a growth factor, or another ligand, the fusion protein may act as a highly specific bifunctional linker. This linker may act to bind and enhance the interaction between cells or cellular components to which the fusion protein binds. Thus, for example, where the fusion protein is a growth factor joined to an antibody or antibody fragment (e.g. an Fv fragment of an antibody), the antibody may specifically bind antigen positive cancer cells while the growth factor binds receptors (e.g., IL2 or IL4 receptors) on the surface of immune cells. The fusion protein may thus act to enhance and direct an immune response toward target cancer cells.
Ligands are typically employed in fusion proteins to act as specific targeting moieties. Generally it is desirable to increase specificity and affinity and decrease cross-reactivity of the fusion protein to make it more effective. For example, native PE and DT are highly toxic compounds that typically bring about death through liver toxicity. PE and DT can be transformed into chimeric toxins by removing the native targeting component of the toxin and replacing it with a different specific targeting moiety (e.g. IL4 which targets cells bearing IL4 receptors). However, even these chimeric toxins show some non-specific binding. They attack the liver in addition to their target cells and, when given in large doses, may also produce death due to liver toxicity.
It has been observed that growth factors, and other targeting moieties, frequently show lower specificity and affinity for their targets when they are incorporated into fusion proteins. See, for example, Debinski, et al., J. Biol. Chem., 268: 14065-14070 (1993); Lorberboum-Galski, et al., J. Biol. Chem., 263: 18650-18656 (1988); Williams, et al., J. Biol. Chem., 265: 11885-11889 (1990); and Edwards, et al. Mol. Cell. Biol., 9: 2860-2867 (1989).